Soluble Immune Checkpoints in Intra-Abdominal Infections

The abdominal region is the second most common source of sepsis and secondary peritonitis. The most frequent causes of abdominal infections are perforation, ischemic necrosis, or penetrating injuries to intra-abdominal organs. Management consists of infection source control, restoration of gastrointestinal (GI) function, systemic antimicrobial therapy, and support of organ function. Despite advances in care, mortality following secondary peritonitis remains high. Excluding patient-related factors such as age or comorbidities that cannot be influenced during intervention, delays in surgical intervention and failure to achieve source control are the main determinants of outcomes.

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection and accounted for approximately 11 million deaths worldwide in 2017, representing 19.7% of all global deaths. Early deaths in sepsis are typically due to septic shock caused by a cytokine storm. In contrast, late deaths result from the inability to clear primary infections and the development of secondary infections due to a state of immunosuppression.

Multiple mechanisms contribute to immune dysfunction in sepsis. Immune checkpoints, which assist in T cell activation, play critical roles in this process. Immune checkpoint molecules expressed on lymphocytes and antigen-presenting cells (APCs) - including CD28, cytotoxic T-lymphocyte antigen 4 (CTLA-4), CD80, CD86, programmed death-1 (PD-1), and programmed death-ligand 1 (PD-L1) - as well as CD40 and CD40L, OX40 and OX40L, 4-1BB and 4-1BBL, B and T lymphocyte attenuator (BTLA), and the T cell immunoglobulin mucin (Tim) family, serve as co-signaling molecules for T cell activation and immune regulation. Immune checkpoints exert a dual effect on the host response during sepsis. On one hand, during pathogen invasion, immune checkpoints provide critical secondary signals that help APCs activate T cells. Activated T cells and APCs create a positive feedback loop, triggering the cytokine storm. On the other hand, immune checkpoints also regulate T cell activation and inhibition in the late phase of sepsis, leading to T cell anergy and apoptosis.

CTLA-4, also known as CD152, is a competitive receptor for CD28 and plays a negative regulatory role in the immune response of inflammatory diseases by modulating CD28-mediated T cell costimulation. CTLA-4 is also expressed on T regulatory (Treg) cells and temporarily blocks their immune effects. Anti-CTLA-4-based immunotherapy has shown a dose-dependent effect in reducing sepsis-induced apoptosis in a cecal ligation and puncture (CLP) mouse model of sepsis. However, it has minimal effects on inflammatory cytokines.

PD-1, an inhibitory receptor of the CD28 family, is mainly expressed on lymphocytes, dendritic cells (DCs), monocytes, and macrophages. PD-L1, another member of the B7 family and the primary ligand of PD-1, can also be found on lymphocytes, DCs, monocytes, and macrophages. PD-1 and PD-L1 play critical roles in the immunosuppressive state of sepsis and may serve as valuable tools for assessing immune status and as potential therapeutic targets in sepsis. Various studies have confirmed that PD-1, PD-L1, and PD-L2 expression on monocytes and T lymphocytes is increased in septic patients, contributing to a novel immune regulatory system involved in immune dysfunction during sepsis. PD-1 expression on monocytes and T cell repertoire diversity have shown a positive correlation with serum interleukin levels and have been predictive of mortality in patients with septic shock. PD-L1 expression on neutrophils and monocytes has also been associated with risk stratification and mortality in septic patients.

For these reasons, elucidating the roles of immune checkpoints in the pathogenesis of sepsis and the use of immunotherapy hold great potential for sepsis treatment. Although there are limited studies in the literature on the expression of PD-1, PD-L1, PD-L2, and CTLA-4 on T cells and monocytes in sepsis patients, no studies have focused on these markers specifically in intra-abdominal infection patients or on their soluble forms in serum.